Sounds to me that you have dinoflagalletes rather than a diatom problem since you mentioned they appear only when the light is on. Controlling the nutrient level in the tank should do the trick in bringing this infestation under control.
Mainly bring down nitrate,phosphates and silicates.
Some ways would be:
- Avoid overfeeding (not guilty)
- Top-up with RO/DI water instead of tapwater (guilty, but have been doing it for past 2yrs, so unlikely cause)
- Do regular water changes using RO/DI water ideally (water change 15% weekly for past 1.5yrs, now 30% eevry 2 weeks)
- Use chemical media to remove phosphates/silicates chemically, if possible use it in a fluidized reactor for best results (guilty, recently stopped using carbon & phosguard, most likely cause)
Thanks bro kalib!